1. Field of the Invention
The present invention relates to an image processing device, an image processing method and an image processing program product and, more specifically, to an image processing device, an image processing method and an image processing program product that are capable of high quality color output of digital data.
2. Description of the Related Art
In an image forming device such as a 4-cycle image forming device or a color tandem image forming device, when digital data of an electronic photograph, for example, is to be output in color, a multi-colored image is formed by superposing presses corresponding to toners of cyan (C), magenta (M), yellow (Y) and black (K), as basic colors, respectively.
More specifically, in a color tandem image forming device, for example, four image forming units each including a developer are arranged along an intermediate transfer belt, presses as toner images of respective colors formed by respective units are transferred onto the intermediate transfer belt (primary transfer), and by the superposition of the presses, a multi-colored image is formed. Then, the image formed by registration on the intermediate transfer belt is transferred onto a sheet of paper as a recording medium (secondary transfer), and after a fixing step, the result is output.
When the color image to be output has an image of K component only (K-image) arranged on a color background image including (at least one of) CMY components, a white gap may appear at the boundary between the background and the K-image.
Further, even when the image arranged over the background is not only of the K component but a gray image of low chroma formed of CMYK components and the CMYK components of the background image portion and the CMYK components of the gray image portion may have some overlapping CMYK component, a similar gap, though slightly colored with toner but almost white, may appear at the boundary between the background and the gray image, because of mis-registration of presses, if the amount of overlapping component is small.
In order to prevent such a gap, a process called “overprinting” has been used, in which a K-image or a gray image is arranged on top of the background image to fill also the K-image portion with the background color. Conventionally, overprinting is done simply by superposing a K-image formed of K component only on top of the background color, or by superposing a K-image formed of K component with the background color made thinner, as disclosed in Japanese Laid-Open Patent Publication No. 2004-174964.
When overprinting is done, however, unwanted color mixing of the single K component of the K-image and the background color occurs, changing the color shade of gray, so that the color of single K component comes to be different from the color in the overprinted area. Though the background color is made thinner in the method disclosed in the laid-open application mentioned above, it is still true that a different color shade is added as the background color, and color shift always occurs by superposing, unless the background color is of low chroma.